On Nov. 11, 2013, Victorville, California, sheriff’s deputies and a coroner responded to a motorcyclist’s report of human remains outside of town.

They identified the partially bleached skull of a child, and later discovered the remains of the McStay family who had been missing for the past three years. Joseph, 40, his wife Summer, 43, Gianni, 4, and Joseph Jr., 3, had been bludgeoned to death and buried in shallow graves in the desert.

Investigators long suspected Charles Merritt in the family’s disappearance, interviewing him days after they went missing. Merritt was McStay’s business partner and the last person known to see him alive. Merritt had also borrowed $30,000 from McStay to cover a gambling debt, a mutual business partner told police. None of it was enough to make an arrest.

Even after the gravesite was discovered and McStay’s DNA was found inside Merritt’s vehicle, police were far from pinning the quadruple homicide on him.

Until they turned to Project Hemisphere.

Hemisphere is a secretive program run by AT&T that searches trillions of call records and analyzes cellular data to determine where a target is located, with whom he speaks, and potentially why.